Several different amplifier applications require an amplifier having a large gain-bandwidth product. For example, RF signals on optical fibers may require large gain-bandwidth product amplifiers that are highly linear. Some broadband fiber and RF communications applications may require large gain-bandwidth product amplifiers to provide high spectral efficiency. Software configurable communications systems may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth, which may span baseband frequencies to microwave frequencies. Baseband to microwave instrumentation may require an amplifier having a large gain-bandwidth product and a very wide operating bandwidth.
Distributed amplifiers (DAs) typically utilize multiple transconductance elements coupled in series to provide an amplifier having a larger gain-bandwidth product than is possible with an amplifier using a single comparable transconductance element. A DA may have an input line of inductive elements or transmission line segments coupled in series and a parallel output line of inductive elements or transmission line segments coupled in series. The input and the output lines have corresponding taps that are coupled to the multiple transconductance elements, such that an input signal, which is applied to one end of the input line, propagates down the input line. As the input signal propagates down the input line, each successive transconductance element receives and amplifies the input signal to feed a corresponding tap into the output line. Each successive transconductance element adds to the amplified input signal. As such, the amplified input signal propagates down the output line to provide an output signal at the end of the output line. Ideally, the input line and the output line have identical delays, such that the input signal and the amplified input signal stay in phase with one another so that each transconductance element adds to the amplified input signal in phase. However, practical DAs may have phase velocity variations, distortions, or both along the output line that may degrade the linearity of the DA, the efficiency of the DA, or both.
Capacitively-coupled DAs may be used to extend gain-bandwidth products of the DAs. However, capacitive-coupling may limit low frequency operation of the DAs. Thus, there is a need for a capacitively-coupled DA having a large gain-bandwidth product, having a very wide operating bandwidth, and that compensates for phase velocity variations along the output line to maximize linearity and efficiency.